Light collector for photostimulable phosphor imaging systems

ABSTRACT

Apparatus for collecting and detecting radiation emitted by, reflected from, or transmitted through a scanned information medium. The apparatus includes a photodetector assembly and first and second planar mirror assemblies configured to maximize collection efficiency and flare radiation attenuation.

FIELD OF THE INVENTION

This invention relates to an apparatus for reading the image stored in aphotostimulable storage phosphor. More particularly, this inventionrelates to an apparatus for collecting and detecting the radiationemitted from a photostimulable storage phosphor during scanning bystimulating radiation. This apparatus can also be utilized in imageacquisition systems in which the image bearing media is either diffuselyreflective or diffusely transparent.

BACKGROUND OF THE INVENTION

Specularly reflective collectors which have been designed to date havenot been well optimized in terms of both energy collection efficiencyand control of flare radiation. This is due partially to the fact thatthere are no commercially available computer programs capable ofoptimizing such designs. The designer must rely on conceptual designsthat can only be analyzed on a computer. In addition, there has not beena strong emphasis placed on maximizing collection efficiency, rather;much of the past effort would appear to have been directed towardminimizing flare radiation. Flare radiation is defined as that portionof the stimulating radiation, reflected or scattered by the storagephosphor, which enters the collector and propagates along a path, suchthat it exits the collector and strikes the storage phosphor at aposition which does not coincide with the position of the scanning beam.This errant radiation will stimulate blue photon emissions from thisother location and thereby corrupt the signal which is being detectedsynchronously with the position of the scanning beam, as well as thesignal which will be detected from this other location if it has not yetbeen scanned. The net effect of flare radiation is to corrupt thefidelity of the latent image by reducing the overall dynamic range, andin particular, the contrast ratios in regions of low exposure.

When considering specularly reflective collector designs, the designershould be aware of a few important guidelines. First, the number ofreflections required to reach the detector must be minimized in order toachieve high collection efficiency. For example, an aluminum reflectorwill absorb 8% of the incident 390 nm emission with each reflection.Second, referring to FIG. 1, the declination angle, α, of the detector4, with respect to the surface normal 2 of the storage phosphor 1,should be minimized. Given that the emissions are Lambertian in nature,the radiated energy which strikes the detector directly, withoutundergoing any reflection, is proportional to the cosine of thisdeclination angle (i.e., the projected area of the source). Third, thecross sectional angle, ω, subtended by the detector 4 should be kept aslarge as possible to maximize collection of the radiated energy whichcan strike the detector directly. This can be achieved by keeping β, theangle that the detector's mirror 3 makes with respect to the line ofsight, as close to zero as possible, thus maximizing the projected areaof the detector as viewed from the phosphor. In addition, the crosssectional angle, ω, can be increased by minimizing the distance betweenthe detector and the phosphor. Fourth, the detector may reflect a largeportion of the incident energy, hence, the collector should be designedto return as much of this energy as possible back to the detector, witha minimal number of reflections. In the case of a photomultiplier tubewith a K₂ CsSb photocathode, the blue radiation reflectivity has beenshown to be approximately 22%. And lastly, the design must prevent orminimize flare radiation.

U.S. Pat. No. 4,742,225 discloses a reflective collector design withelliptical cross section possessing very good collection efficiency. Inthis design, the detector's declination angle is approximately 20degrees, and the cross sectional angular subtense of the detector isapproximately 26 degrees. A closed form solution of the flux incident onan array of detectors at this location and orientation shows thatapproximately 20% of the emitted radiation will strike the detectorarray directly. In addition, the emitted radiation which fails to strikethe detector directly, will strike it after a single reflection. Thedesign has the following disadvantages. First, fabrication of theelliptical reflector can be difficult or costly. Second, no attempt hasbeen made to recycle the energy reflected by the detector; rather theback side of the entrance aperture is made into an absorber to minimizeflare radiation. If this mirror were made of a specularly reflectivematerial, the bulk of the energy reflected by the detector would require2 to 3 reflections to reach the detector again. Third, flare radiationis not as well controlled as in other collector designs. (See thefollowing U.S. patents which disclose systems having these, as well asother disadvantages: U.S. Pat. No. 3,663,083, issued May 16, 1972,inventors Friedman et al; U.S. Pat. No. 4,346,295, issued Aug. 24, 1982,inventors Tanaka et al; U.S. Pat. No. 4,736,102, issued Apr. 5, 1988,inventor Morrone; U.S. Pat. No. 4,775,791, issued Oct. 4, 1988,inventors Owen et al.)

U.S. Pat. No. 4,743,758 discloses three specularly reflective boxdesigns. In these designs, the detector's declination angle varies from49 to 53 degrees. In addition, the detector is located at such adistance from the phosphor that it subtends a small cross sectionalangle of only 11 to 16 degrees. At such a location, much of the emittedradiation must undergo numerous reflections in order to reach thedetector. A closed form solution of the flux incident on an array ofdetectors at this location and orientation shows that approximately 7%of the emitted radiation will strike the detectors directly. The chiefadvantages of the reflective box designs are that flare radiation iswell controlled, and the mirrors are relatively inexpensive tomanufacture. The chief drawback of these designs is a lower collectionefficiency due to: the small angular subtense of the detector, thenumber of reflections required to reach the detector, and the inabilityto recycle the reflected energy.

U.S. Pat. Nos. 4,743,759, 5,105,079, 5,134,290, and 5,140,160 disclosevarious designs regarding the use of tapered roof mirrors to direct thestimulated emissions towards the detector as well as to direct thescattered stimulating radiation away from the phosphor. These designsemploy a very large diameter detector to increase the angular subtenseof the detector and to increase the angle of the taper. However, thelocation and orientation of the detector with regard to the position ofthe stimulated emissions force these designs to rely heavily uponmultiple reflections. Collection efficiency suffers because many of theparameters affecting the angular subtense of the detector become selfdefeating in these designs. For instance, the detector's declinationangle, α, is approaching zero degrees just as the angle that thedetector's mirror normal makes with respect to the line of sight, β, isapproaching 90 degrees, and vice versa. Likewise, the distance betweenthe detector and the emission source decreases as the angle that thedetector's mirror normal makes with respect to the line of sight, β, isincreasing. In addition, the vertically oriented mirror essentiallydoubles the size of the upper entrance aperture, thereby allowing twiceas much energy to escape from the collector. This results in a greatreduction of collection efficiency near the far ends of the collectorwhere this aperture is closest to the phosphor. The chief advantages ofthe roof mirror configurations are the low cost associated withutilization of a single detector and plane reflectors. In addition,flare radiation is extremely small.

U.S. Pat. No. 4,849,632, issued Jul. 18, 1989, inventor Watanabe; U.S.Pat. No. 4,591,714, issued May 27, 1986, inventors Goto et al; and U.S.Pat. No. 4,591,715, issued May 27, 1986, inventor Goto disclosestimulable phosphor imaging systems in which emitted light iscollectedly a solid transparent light guide assembly and detected bymultiple photomultiplier tubes. The light guides disclosed in thesepatents are expensive, difficult to manufacture and rely on multiplelight reflections to the PMTs, thus reducing light collectionefficiency.

There is thus a problem in the prior art of providing a light collectorin stimulable phosphor imaging systems which has high light collectionefficiency and flare radiation attenuation.

SUMMARY OF INVENTION

According to the present invention, there is provided a specularlyreflective collector which solves the problems of the prior art and inwhich the collection efficiency and flare radiation attenuation exceedsthose performance characteristics of the prior art.

This collector achieves a very high level of performance for thefollowing reasons. First, all stimulated emissions strike the detectoreither directly or within a single reflection--except for a very smallpercentage of those emissions which strike the end reflectors of thecollector housing. The height of the detector array was optimized tomaximize the amount of stimulated emissions that strike the detectordirectly, within the mechanical constraints of the storage phosphorscanner system in which the collector will be utilized. Analysis showsthat 26% of the emitted radiation will strike the detector arraydirectly. The location and orientation of a first set of planarreflectors are chosen such that each will reflect incident emittedradiation directly onto the detector array with a single reflection.Second, the location and orientation of a second set of planarreflectors reflect that radiation, that is reflected rather thanabsorbed by the detector's photocathode, back onto the detector a secondtime with only a single reflection--except for that small percentage ofemissions that strike the end reflectors of the collector housing. Byrecycling this reflected energy with only a single reflection loss,collection efficiency can be increased 20 percent. The "unique"recycling property of the collector design of the present invention alsoforces the small percentage of stimulating radiation reflected by thestimulating radiation absorption filter to be directed back onto thisfilter a second time. A second pass at this filter is all that isrequired to extinguish flare radiation. Analysis of the collector,according to the invention, reveals that 77% of the stimulated emissionsreach the detector. Note, with an entrance aperture of 0.250 inches inwidth, located at a height of 0.0625 inches, only 89% of the stimulatedemissions even enter the collector housing. Of that portion of thestimulated emissions that enter the collector, 87% of that radiationwill reach the detector. The portion of the stimulating radiation thatis returned to the phosphor (flare radiation) is less than 0.1 percent.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a generic collector's cross-sectional geometricrelationships.

FIG. 2 is a perspective view of a storage phosphor scanner including acollector according to the present invention.

FIGS. 3-7 are diagrammatic cross-sectional views of the collector ofFIG. 1 which are useful in explaining the present invention.

FIG. 8 is a graphical view of collector efficiency as a function oflaser beam scan position for the collector of FIG. 2.

FIG. 9 is a graphical view of flare energy as a function of laser beamscan position for the collector of FIG. 2.

FIGS. 10-17 are cross-sectional diagrammatic views useful in explainingthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 illustrates the basic components of a storage phosphor scanner.Stimulation radiation is provided by a laser source 16. The laser beamis shaped by beam shaping optics 15 and is then caused to raster scan byreflection from a galvanometer 14 or other suitable scanning mechanism.

The beam then passes through an F-theta lens 17 to linearize beamposition on the storage phosphor 9 with the angular position of the scanmirror. The scanning laser beam 11 then passes into the collector 100through a first slit aperture 10 which lies between specularlyreflective mirrors 2 and 3, and immediately exits the collector 100through a second slit aperture 12 (FIG. 3) at the bottom of thecollector 100. Upon exiting the second aperture 12 in the collector 100,the beam is brought to a focus on the storage phosphor 9 creating bothstimulated emissions from a very small pixel area on the storagephosphor 9 and a large amount of scattered stimulating radiation. Thelaser beam scans the width of storage phosphor 9 as it is moved in adirection perpendicular to the scanning direction. Thus, storagephosphor 9 is scanned in a raster pattern.

Approximately 89%, of the stimulated radiation which is released inproportion to the number of x-ray absorptions at that particular photosite and the intense scattered stimulating radiation, enter thecollector 100 through second slit aperture 12. A small portion,approximately 5% of this stimulated radiation escapes out of thecollector through first slit aperture 10.

The remainder of the stimulated radiation is directed onto thestimulating radiation absorption filter 7 and the five photomultipliertubes (PMT) 8 either directly, or indirectly through reflection frommirrors 1, 2, or 3. A small percentage of the stimulating radiation isreflected by the front mirror of the stimulating radiation absorptionfilter 7. A much larger percentage of the stimulated radiation isreflected by the PMTs 8. These reflected radiations are returned back tothe stimulating radiation absorption filter 7 and the PMTs 8 by a singlereflection from mirrors 5 or 6. By recycling these radiations one moretime with a single reflection, collection efficiency is very nearlymaximized and flare radiation is essentially removed from the system.The detected signal and the position of the scanning beam are recordedto produce a digital representation of the latent x-ray image recordedby the storage phosphor.

The PMTs 8, as an example, are comprised of five 3 by 3 inch squarephotomultiplier tubes 8 (Reference FIGS. 2 and 3) of the Burle C83079Eor Hamamatsu R4445 models. The height of the photocathode of PMT 8 islocated approximately 2 inches above the storage phosphor 9, and isdisplaced laterally from the scanning laser beam 11 by approximately2.125 inches for mechanical clearance. The height of the photocathode ofPMT 8 was optimized to maximize the collection of stimulated emissionsthat can strike the PMT 8 directly, at such a lateral displacement.

FIG. 3 details the direct ray paths that emissions follow to the PMT 8.For the stimulated radiation that strikes the detector between points"A" and "C", the portion that is reflected by the detector will strikemirror 5 between points "G" and "H", reflect, and strike the PMT 8 againbetween points "D" and "E" For the stimulated radiation that strikes thePMT 8 between points "C" and "F", the portion that is reflected by thePMT 8 will strike mirror 6 between points "H" and "I", reflect, andstrike the PMT 8 again between points "B" and "F".

FIG. 4 details the limiting indirect ray paths that emissions followfrom a single reflection at mirror 1 to the PMT 8. For the stimulatedradiation that strikes the PMT 8 between points "A" and "C", the portionthat is reflected by the PMT 8 will strike reflective mirror 5 betweenpoints "G" and "H", reflect, and strike the PMT 8 again between points"D" and "E". For the stimulated radiation that strikes the PMT 8 betweenpoints "C" and "E", the portion that is reflected by the PMT 8 willstrike reflective mirror 6 between points "H" and "I", reflect, andstrike the PMT 8 again between points "B" and "F"

FIG. 5 details the limiting indirect ray paths that emissions followfrom a single reflection at mirror 2 to the PMT 8. For the stimulatedradiation that strikes the PMT 8 between points "A" and "B", the portionthat is reflected by the PMT 8 will strike reflective mirror 5 betweenpoints "G" and "H", reflect, and strike the PMT 8 again between points"C" and "F". For the stimulated radiation that strikes the PMT 8 betweenpoints "B" and "D", the portion that is reflected by the PMT 8 willstrike mirror 6 between points "H" and "I", reflect, and strike the PMT8 again between points "B" and "E".

FIG. 6 details the limiting indirect ray paths that emissions followfrom a single reflection at mirror 3 to the PMT 8. For the stimulatedradiation that strikes the PMT 8 between points "A" and "B", the portionthat is reflected by the PMT 8 will strike mirror 5 between points "G"and "H", reflect, and strike the PMT 8 again between points "E" and "F".For the stimulated radiation that strikes the PMT 8 between points "B"and "D", the portion that is reflected by the PMT 8 will strike mirror 6between points "H" and "I", reflect, and strike the PMT 8 again betweenpoints "C" and "E".

No radiation reflects from member 4 (which may or may not be a mirror).The slit aperture 12 could be increased to allow more stimulatedemissions to enter the collector 100 by shortening the cross sectionallength of member 4 and increasing the angle of inclination that it makeswith the storage phosphor 9. In addition, the member 4 would have to bemade reflective. The additional gain in performance though would berather marginal--at a maximum, collection efficiency could be increasedfrom 77% to approximately 81%.

AS shown in FIG. 7, for situations in which mechanical space constraintsmay not allow the PMT 8 array to be oriented vertically, this collectorconfiguration can essentially be flipped about a 45 degree diagonal tocreate a collector 100 with a horizontally mounted PMT 8 array that hasproperties identical to those possessed by the previously describedcollector 100, but with only a very slight loss in collection efficiencydue to a minuscule reduction in stimulated emissions which can strikethe detector directly (23% vs. 26% previously), and a correspondingincrease in stimulated emissions that must reflect once off mirrors 1,2, or 3 prior to reaching the PMT 8. FIG. 7 details the direct ray pathsthat emissions follow to the PMT 8. For the stimulated radiation thatstrikes the PMT 8 between points "A" and "C", the portion that isreflected by the PMT 8 will strike mirror 5 between points "G" and "H",reflect, and strike the PMT 8 again between points "D" and "F" For thestimulated radiation that strikes the PMT 8 between points "C" and "E",the portion that is reflected by the PMT 8 will strike mirror 6 betweenpoints "H" and "I", reflect, and strike the PMT 8 again between points"B" and "F".

Similar diagrams as above can be shown which illustrate the limitingindirect ray paths to the PMT 8, reflecting from the PMT 8, and beingdirected back to it with a single reflection from mirror 5 or 6, for thethree scenarios of initial reflection from mirrors 1, 2, or 3.

Collector Performance

FIG. 8 is a plot of the collection efficiency for this particularcollector 100. This analysis assumes that the collector's reflectivemirrors of mirrors 1, 2, 3, 5, and 6 are 92% reflective (8% absorptive)and the PMT 8 surface was 22% reflective (78% absorptive). FIG. 9 is aplot of the flare radiation for this particular collector 100. Thisanalysis assumes that the collector's 100 reflective mirrors are 92%reflective (8% absorptive) and the PMT 8 surface was 4% reflective (96%absorptive). The reflectance of the PMT 8 is less in this case becausethe stimulating radiation absorption filter 7 essentially attenuates allstimulating radiation not reflected by the first surface of this filter7.

Collector Design Philosophy of an Example

For an example, the minimum cross section width of the surface of thePMT 8 is 2.75 inches. Mechanical constraints relating to the storagephosphor reader in which this collector 100 is utilized require thecenter of the PMT 8 to be located no closer than 2 inches to the scanbeam, and it's surface oriented nearly parallel to the surface of thephosphor 9. The PMT 8 surface therefore becomes a 3 by 15 inchrectangular aperture, located 2 inches off center relative to theemission point. The height of the detector's surface, approximately 2inches, was optimized with regard to maximizing the detection ofdirectly collected emissions.

The portion of the power emitted from a small Lambertian source that iscollected by a rectangular aperture centered over the source is given by##EQU1## where "l" is the half length of the rectangular aperture, "w"is the half width of the rectangular aperture, and "h" is the height ofthe aperture above the source. This equation and it's derivation can befound in H. Cotton's Principles of Illumination (John Wiley, New York,1960, p. 157) and P. Moon's The Scientific Basis of IlluminatingEngineering, 2nd Edition (Dover, New York, 1961, p. 267). In thesetexts, the authors actually derive the irradiance at a position due to alarge rectangular Lambertian source; however, the mathematics is forthis application is identical.

In order to derive the optimum height for an off axis rectangularaperture, one must first compute the portion of the flux subtended by arectangular source whose half width is equal to the actual half width ofthe aperture plus the amount by which the aperture is decentered. Nextone must compute the portion of the flux subtended by a rectangularsource whose half width is equal to the amount by which the aperture isdecentered minus the actual half width of the aperture. The result ofthis second calculation must be subtracted from the result of the first,and the answer divided by two. Repeating this procedure for variousheights of the aperture enables one to numerically solve for the optimumheight. For this particular configuration, the optimum height isapproximately 2 inches, at which height a minimum of 28% of the emittedphotons strike the PMT 8 directly without any intervening reflections.

With regard to those photons that do not strike the PMT 8 directly; thesize, location, and angular orientation of mirrors 1, 2, and 3 have beenchosen such that emissions striking these mirrors will be directed toPMT 8 without requiring any additional reflections. This task can beaccomplished by utilizing a single cylindrical reflector (Reference FIG.10) whose cross section resembles a conic section (i.e., a parabola, anellipse, a hyperbola, or a circle); however, these surfaces can bedifficult, or at least costly to manufacture. The end result ofutilizing conic section reflector can also be achieved by utilizingproperly sized, located, and oriented planar reflective surfaces. Thelatter being much more economical to fabricate.

FIG. 11 is a cross section of the collector 100 which shows the relativesize, location, and orientation of mirror 1. Mirror 1 is orientedparallel to the scan path and perpendicular to the phosphor 9. Thebottom edge of mirror 1 is chosen to be the same height above thephosphor 9 as the bottom of mirror 4. In this particular embodiment, thelower height is 1/16th of an inch, to accommodate warpage of thephosphor 9. The bottom edges of mirrors 1 and 4 form an aperture 12 of1/4 of an inch in width. This aperture 12 allows 89.4% of the photonsemitted to enter the collector 100. Emissions directed at the bottomedge of mirror 4, enter the collector 100 and propagate along mirror 4striking the edge of the absorption filter 7 at point F. This path isparallel to both mirror 4 and that path followed by emissions thatstrike the bottom edge of mirror 1 and are directed to point E at theabsorption filter 7. The height of the top edge of mirror 1 is definedby the intersection of mirror 1 and the emission path that is directedto the edge A of the absorption filter 7's mirror image. If the top edgeof mirror 1 were any higher, those photons intercepted by the extensionof this surface could not reach the PMT 8 with a single reflection. Forthis particular embodiment, the height of mirror 1 above the phosphor 9is approximately 0.22 inches.

FIG. 12 is a cross section of the collector 100 which shows the relativesize, location, and orientation of mirror 2. Mirror 2 is orientedparallel to the scan path. The bottom edge of mirror 2 is coincidentwith the top edge of mirror 1. The angular orientation of mirror 2 isadjusted such that photons striking the bottom edge of mirror 2 aredirected to the edge F of the absorption filter 7. This occurs when thestraight line extension of this emission path intersects the edge F ofthe absorption filter 7's mirror image. The height of the top edge ofmirror 2 is defined by the intersection of mirror 2 and the emissionpath that is directed to the edge A of the absorption filter 7's mirrorimage. For this particular geometry, the height of mirror 2 above thephosphor 9 is approximately 0.71 inches. In addition, mirror 2 has beenrotated approximately 18.8 degrees relative to mirror 1. With regard tothe actual embodiment, the height of mirror 2 above the phosphor 9 wasadjusted to approximately 0.57 inches, and the angular rotation wasreduced to approximately 15.2 degrees. At first glance, this appears tobe a major departure from the original design philosophy; however, thefollowing explanation shows that not to be the case.

If mirror 2 were terminated at a height of 0.71 inches with a rotationangle of 18.8 degrees, it would extend into and slightly to the right ofthe scanning beam. When the mirror 2 is slotted, to allow the scanningbeam to pass through the collector 100, mirror 2 would become twoseparate mirrors--a large mirror on the left side of the scanning beamand an extremely small one on the right side of the scan beam. Insteadof constraining the rotation of this mirror so that photons striking thebottom edge of mirror 2 are directed toward edge F of the absorptionfilter 7, the rotation of this mirror 2 is constrained to direct photonsstriking the top edge of mirror 2, toward edge A of the absorptionfilter 7 (Reference FIG. 5). The top edge of mirror 2 is now beingdefined by its intersection with an imaginary plane declined 2.86degrees to the left of phosphor 9's mirror normal and coincident withthe scan line. Two imaginary planes, declined 2.86 degrees to the rightand left of phosphor 9's mirror normal, and coincident with the scanline, define a triangular region in which 5.0% of all photons emittedfrom the phosphor 9 can escape through the upper slot 10. A 5.0% lossthrough slot 10 was established as a baseline in the design of thiscollector 100. Mirror 2, with it's original 18.8 degree rotation, couldhave been terminated on the left side of phosphor 9's mirror normal, atthe intersection of mirror 2 and the imaginary plane--a height of 0.51inches. The result would have been emissions striking mirror 2 would bereflected toward the right side of the PMT 8 at a higher angle ofincidence. By deviating slightly from the general design philosophy,emissions striking this mirror are now reflected toward the left side ofthe PMT 8 (Locations A through D--Reference FIG. 5) at a lower angle ofincidence, thereby decreasing the reflectance at the absorption filter7. In addition, the upper entrance slot 10 is now 12% higher than itwould have been above the phosphor 9, and it's width is increased by12%--making alignment easier--with no increase in lost photons. Thisslight variation in the design philosophy was made possible because, forthis particular geometry, emissions striking mirror 3 do not fill all ofPMT 8 as will be seen below; therefore, there is room for some "give andtake" between the relative sizes and angular orientations of mirrors 2and 3. Because of the fact that an extension of mirror 2 does notintersect the PMT 8, a third mirror is required.

FIG. 13 is a cross section of the collector 100 which shows the relativesize, location, and orientation of mirror 3. Mirror 3 is orientedparallel to the scan path. The bottom edge of mirror 3 is coincidentwith the top edge of mirror 2. Following the same design philosophydescribed above, the angular orientation of mirror 3 would be adjustedsuch that photons striking the bottom edge of mirror 3 are directed tothe edge F of the absorption filter 7. Given this orientation, mirror 3would intersect PMT 8 at a position other than location A, therebyobstructing a portion of the PMT 8. Hence, the angular orientation ofmirror 3 is adjusted to intersect the PMT 8 at location A. In thisangular orientation and location, mirror 3 is parallel and coincidentwith the path taken by photons that strike the top edge of mirror 2 ontheir way to location A of the PMT 8. For this particular embodiment,that rotation angle is approximately 33.3 degrees relative to mirror 1.This angular orientation redirects emissions that strike the bottom edgeof mirror 3 away from location F to a new location about 1/3 of the waytowards location D of absorption filter 7. This orientation of mirror 3has the additional benefit of reducing the angle of incidence of theemissions upon the detector, thereby decreasing the reflectance atabsorption filter 7.

In order to allow an opening for the scan beam to pass through thecollector 100, the bottom edge of mirror 3 is trimmed away to coincidewith the intersection of mirror 3 and the imaginary plane that is tilted2.86 degrees to the right of phosphor 9's mirror normal that iscoincident with the scan beam. The extension of mirror 3 still coincideswith the top edge of mirror 2; however, the actual height of the bottomedge of mirror 3 is now approximately 0.67 inches for this particularembodiment. The height of the top edge of mirror 3 is defined by theheight of absorption filter 7, which is approximately 1.75 inches abovethe phosphor 9.

By redirecting all intersecting emission paths toward PMT 8 with asingle reflection, this clever sizing and arrangement of mirrors 1, 2,and 3 also prevents any photons from escaping through the upper slot 10,other than by direct emission path. Many collector designs in the priorart fail in this regard. In those designs, an image (s) of the upperslot is visible from the emission point, in the reflective mirrors;thereby providing additional emission paths out of the collector andlowering collection efficiency.

FIG. 14 is a cross section of the collector 100 which shows the relativesize, location, and orientation of mirror 5. Mirror 5 is orientedparallel to PMT 8. The left edge, G, and it's height above the phosphor9 is defined by the intersection of the reflection path (A to G) forphotons that strike the PMT 8 directly at location A (any other emissionpath striking PMT 8, whether directly or indirectly, will reflect andstrike mirror 5 to the right of location G), and the path followed bythose photons that strike PMT 8 directly at location F. For thisparticular embodiment, mirror 5 is approximately 0.61 inches abovephosphor 9, and it's left edge is located approximately 1.23 inches tothe right of the scan beam. The combination of mirror 5 with PMT 8 forma channel in which the photons, which must enter from the lower left,will enter and reflect back and forth along a path similar to thatillustrated in FIG. 14 (A-G-D-H-E) as the energy propagates down thechannel. Mirror 5 thus returns that energy reflected by the PMT 8 backto PMT 8 with a single reflection. Because of the finite extent of thedetector 100, the length of this channel must be terminated with anothermirror--mirror 6.

Referring back to FIG. 6, there exists a ray path leading from theemission point, to mirror 3, on to absorption filter 7 at location B, onto mirror 5 at location H, and finally reflecting and striking theabsorption filter 7 at it's right most edge--location F. The angularorientation of mirror 6 is such that it intersects/terminates mirror 5precisely where this ray path intersects mirror 5. Thus mirror 6 isparallel and coincident with this ray path. We are most concerned withmirror 3 ray paths because these ray paths have the largest angle ofincidence (reflectance) at mirror 5. If mirror 5 extended any further tothe right of this location H, mirror 3 ray paths that strike mirror 5 tothe right of location H would strike mirror 6 prior to striking the PMT8 thus requiring 2 reflections to return to the PMT 8. Mirror 3 raypaths that would normally strike mirror 5 to the right of location H nowstrike mirror 6 and are returned back to PMT 8 with a single reflection.In this particular embodiment, the right most extent of mirror 5 isapproximately 2.24 inches from the scan beam 11. This intersection pointrequires mirror 6 to be rotated approximately 42 degrees relative tomirror 1.

Advantages

The advantages of this collector design are exceptionally highcollection efficiency, near uniform collection efficiency signature,exceptionally low flare radiation, and low manufacturing costs due tothe utilization of planar reflectors. With regard to specularlyreflective collectors, the collection efficiency has been very nearlyoptimized. Any further significant improvement in collection efficiencywould probably require the utilization of larger mirror area detectorsor utilization of additional detectors, so as to increase the amount ofemitted radiation that can be detected prior to undergoing anyreflections.

The invention has been described in detail herein with reference to thefigures, however, it will be appreciated that variations andmodifications are possible within the spirit and scope of the invention.For example, the invention can be used in other imaging systems in whichan information medium is scanned with a radiation beam to produce aninformation image which is reflected from or transmitted through theinformation medium. Where the information image is transmitted, the slotbetween mirrors 2 and 3 may be omitted.

I claim:
 1. A collector for collecting and detecting radiation emittedby, reflected from, or transmitted through a scanned information mediumcomprising:a photodetector assembly with first and second sides whichextends the width of a scanned information medium and which has adetection surface substantially parallel to or perpendicular to saidinformation medium; a first planar mirror assembly which extendsparallel to said photodetector assembly, which has a lower edge locatedadjacent to said information medium and an upper region adjacent to saidfirst side of said detection surface of said photodetector assembly; asecond planar mirror assembly which extends parallel to saidphotodetector assembly, which has a lower edge located adjacent to saidinformation medium and an upper region adjacent to said second side ofsaid detection surface of said photodetector assembly; wherein saidlower edges of said first and said second planar mirror assemblies arespaced apart and form an aperture for passing radiation emitted by,reflected from, or transmitted through an information medium; andwherein said photodetector assembly, said first planar mirror assemblyand said second planar mirror assembly are configured (1) so thatsubstantially all radiation passing through said aperture strikes saidphotodetector assembly either directly or after a single reflection fromsaid first planar mirror assembly; (2) so that said first planar mirrorassembly reflects incident radiation only once before it strikes saidphotodetector assembly; and so that said second planar mirror assemblyreflects radiation reflected by said photodetector assembly with only asingle reflection back to said photodetector assembly.
 2. The collectorof claim 1 wherein said photodetector assembly includes a contiguousarray of photomultiplier tubes.
 3. The collector of claim 1 wherein saidfirst planar mirror assembly includes first, second and third specularlyreflective planar mirrors extending parallel to a scanning beam ofradiation.
 4. The collector of claim 3 wherein said first specularlyreflective planar mirror of said first planar mirror assembly isoriented perpendicularly to a scanned information medium.
 5. Thecollector of claim 1 wherein said second planar mirror assembly includesfourth and fifth specularly reflective planar mirrors extending parallelto a scanning beam of radiation.
 6. The collector of claim 5 whereinsaid fourth specularly reflective planar mirror of said second planarmirror assembly is oriented in parallel to said photodetector assembly.7. The collector of claim 1 including planar mirrors at either end ofsaid collector located between said first and said second planar mirrorassemblies.
 8. A collector for collecting and detecting stimulatedradiation emitted from a photostimulable storage phosphor medium scannedby a beam of stimulating radiation comprising:a photodetector assemblywith first and second sides which extends the width of a scannedinformation medium and which has a radiation detection surfacesubstantially parallel to or perpendicular to said photostimulablestorage phosphor medium; said photodetector assembly including a filterover said detection surface for passing stimulated radiation to saidradiation detection surface and for preventing passage of stimulatingradiation to said radiation detection surface; a first planar mirrorassembly which extends parallel to said photodetector assembly, whichhas a lower edge located adjacent to said photostimulable storagephosphor medium and an upper region adjacent to said first side of saiddetection surface of said photodetector assembly; a second planar mirrorassembly which extends parallel to said photodetector assembly, whichhas a lower edge located adjacent to said photostimulable storagephosphor medium and an upper region adjacent to said second side of saiddetection surface of said photodetector assembly; wherein said loweredges of said first and said second planar mirror assemblies are spacedapart and form an aperture for passing stimulated radiation emitted bysaid scanned photostimulable storage phosphor medium; and wherein saidphotodetector assembly, said first planar mirror assembly and saidsecond planar mirror assembly are configured (1) so that substantiallyall stimulated radiation passing through said aperture strikes saidphotodetector assembly either directly or after a single reflection fromsaid first planar mirror assembly; (2) so that said first planar mirrorassembly reflects incident radiation only once before it strikes saidphotodetector assembly; and (3) so that said second planar mirrorassembly reflects radiation reflected by said photodetector assemblywith only a single reflection back to said photodetector assembly andwhereby stimulating radiation reflected from said filter is reflectedback to said filter to effectively eliminate flare induced stimulatedradiation.
 9. The collector of claim 8 wherein said photodetectorassembly includes a contiguous array of photomultiplier tubes.
 10. Thecollector of claim 8 wherein said first planar mirror assembly includesfirst, second and third specularly reflective planar mirrors extendingparallel to a scanning beam of stimulating radiation and includesanother aperture aligned with said aperture for passing said scanningbeam of stimulating radiation.
 11. The collector of claim 10 whereinsaid first specularly reflective planar mirror of said first planarmirror assembly is oriented perpendicularly to a scanned medium.
 12. Thecollector of claim 8 wherein said second planar mirror assembly includesfourth and fifth specularly reflective planar mirrors extending parallelto a scanning beam of stimulating radiation.
 13. The collector of claim12 wherein said fourth specularly reflective planar mirror of saidsecond planar mirror assembly is oriented in parallel to saidphotodetector assembly.
 14. The collector of claim 8 including planarmirrors at either end of said collector located between said first andsaid second planar mirror assemblies.